Arbuscular mycorrhiza, but not hydrogel, alleviates drought stress of ornamental plants in peat-based substrate

In a greenhouse study, we examined the application potential of symbiotic arbuscular mycorrhizal fungi (AMF) and water absorbing gel (hydrogel) in planting pot-grown ornamental plants that periodically experience drought stress. We tested the hypotheses that either of these treatments can improve plant growth and flowering under conditions of water deficiency. Three species of ornamental plants - Gazania rigens, Pelargonium peltatum and P. zonale - were grown in pots with peat-based substrate either inoculated or non-inoculated with commercial inoculum of AMF, and either with or without added hydrogel. Generous watering during the first 14 weeks ensured well-grown plants. In the next 15 weeks, plants either were well watered or were periodically exposed to moderate or strong drought stress to an initial wilting point or sufficient to cause strong wilting, respectively, before the substrate was again re-saturated with water. The three tested species differed in their resistance to drought stress, P. peltatum being the most resistant and G. rigens the most sensitive. Strong drought decreased shoot biomass of G. rigens and P. zonale, but it had no effect on shoot biomass of P. peltatum, that even increased flowering. As a response to drought, mycorrhizal colonization of plant roots intensified and AMF significantly increased shoot biomass (and several biometric parameters) of all plant species and promoted flowering of G. rigens and P. zonale under all watering regimes. AMF thus confirmed the hypothesized potential for promoting plant growth under drought. On the other hand, the addition of hydrogel brought no positive growth effect on any plant species. On the contrary, it decreased flowering of all plants and interacted negatively with AMF.